1. Field of the Invention
The present invention relates to elastomers containing inorganic fillers and silane coupling agents. More particularly, the present invention pertains to such elastomer compositions containing inorganic fillers that have improved dynamic and static hardness coupled with desirable strength, wear resistance, fatigue resistance, improved hysteresis, and the like and to pneumatic tires in which these elastomers are used in the tread.
The term “hardness” is used herein in its broadest technical sense and includes Shore A (ASTM D2240) hardness, IRHD (ASTM D1415, ISO standard) hardness, or other common methods of hardness measurement. Alternatively, the modulus of the rubber compound (G′ shear, E′ Extensional) measured under small dynamic strains (anywhere from 0 to 10% strain, but preferably closer to 0% strain) will be referred to herein as hardness.
2. Description of Related Art
The use of organofunctional silanes as coupling agents in silica-filled tire tread compounds results in substantial performance benefits, including lower hysteresis, improved wet and ice traction, and high abrasion resistance. Unfortunately, these improvements in performance are usually accompanied by a loss in dynamic stiffness (hardness) of the filled rubber. Polysulfide silanes currently used in silica-filled tire treads lead to lower hardness of the compound and blocked mercaptosilanes (e.g., 3-octanoylthio-1-propyltriethoxysilane) amplify this effect. Chemical interaction of silanes with the silica surface results in a weaker silica “network”,i.e., an improved dispersion of the silica in the rubber, thereby reducing the thixotropic benefits of the system. The lack of adequate bound rubber and hydrodynamic interactions between the filler and polymer also causes a reduction in hardness of the rubber. Since hardness of the tread compound is a very important parameter that dictates many of a tire's performance properties, an increase in hardness of compounds containing these silanes is necessary.
From the early 1960's to the present, two families of sulfur-containing silanes have been the most common coupling agents to provide reinforcement in mineral-filled rubbers. The first to be introduced, mercapto-functional silanes, are widely used for effective coupling, but tend to create scorchy compounds and often have noticeable odors. The polysulfidic bis-alkoxysilanes, introduced in the 1970's, provided improved processability and odor. The tradeoff was the requirement to use higher loadings in some formulations. Variations of this family with reduced sulfur content have appeared, providing easier compounding, due to reduced sulfur donation in the high temperature non-productive mix stages typically used for silica compounds. The major growth in recent years in silica/silane-reinforced passenger tire tread formulations using up to 100% silica filler has placed new demands on compounding technology.
Much of the art relating to the use of sulfur-containing coupling agents in rubber involves silanes containing one or more of the following chemical bond types: S—H (mercapto) and S—S (disulfide or polysulfide). Mercaptosilanes have offered superior coupling at substantially reduced loadings. However, their high chemical reactivity with organic polymers leads to unacceptably high viscosities during processing and premature curing (scorch). This undesirability is aggravated by their odor. Hence, a compromise must be found between coupling and the associated final properties, processability, and required loading levels. Polysulfidic silane coupling agents provide this optimal balance of processing and performance characteristics to a certain extent.
Voronkov et al. in Inst. Org. Khim., Irkutsk, Russia and U.S. Pat. No. 3,922,436 disclose acylthioalkyl silanes, such as CH3C(═O)S(CH2)1-3Si(OR)3 and HOC(═O)CH2CH2C(═O)S(CH2)3Si(OC2H3)3. In particular, U.S. Pat. No. 3,922,436 discloses organosilanes useful as coupling agents for glass fibers and as flame retardants, formed by reaction of an epoxy, a mercapto, or an amino silane with a carboxylic acid or anhydride, or formed by a Diels-Alder reaction of a halogenated cyclopentadiene with an unsaturated silane.
U.S. Pat. No. 3,957,718 discloses silica-containing mixtures for the improvement of the adhesion of vulcanizable mixtures of natural and/or synthetic rubber to reinforcing fillers or supports of textile and/or metallic fabrics after the vulcanization that substantially consist of (A) active synthetically produced silica or silicates having a specific surface area according to the BET procedure of about 50 to 500 m2/g and an average primary particle size of from about 5 to 100 and at least one of (B) phenolic resin or aminoplast forming components, namely on the one hand phenols and/or amines and on the other hand aldehydes or aldehyde donors, and/or at least (C) one organosilane which can be a bisalkoxysilylalkyl-oligosulfide of a given formula.
U.S. Pat. No. 4,184,998 discloses the bonding of rubber or other plastics to mineral reinforcements, such as silica pigment, that is brought about by a mercapto alkyl silane without production of foul odors from the mercapto compound by preliminary reversible coupling of the mercapto compound with a material capable of addition reaction, such as toluene diisocyanate. U.S. Pat. No.4,519,430 discloses a pneumatic radial tire with a tread having from 1 part by weight of hydrated amorphous fine-particle silica per 3 parts of carbon black tread reinforcing filler, to about 3 parts by weight of the silica to 1 part of the carbon black. The amount of silica present is preferably in the range from about 18 parts to about 50 parts by weight per 100 parts of natural or synthetic rubber. When such a tread is compounded with a mercaptosilane in which the mercapto group is reversibly blocked with an isocyanate group, the tread is said to provide reduced rolling resistance without loss of traction. The blocking group in this case can be an aromatic isocyanate group or any other organic group that can readily un-block under vulcanization conditions.
U.S. Pat. No. 4,942,192 discloses a rubber composition for use in tires, particularly bead filler, comprising 2 to about 30 parts by weight of a particular modified novolak resin having a self-curability based on 100 parts by weight of rubber ingredient.
U.S. Pat. No. 5,266,620 discloses a rubber composition for the manufacture of automotive vehicle tires suitably as a bead filler therefor. The composition comprises specified proportions of carbon black, a modified novolak phenolic resin, hexamethylenetetramine and a polymeric cardanol based on a starting rubber component such that the resulting rubber composition has a relatively low rigidity prior to vulcanization and a relatively high dynamic modulus after vulcanization.
U.S. Pat. No. 5,387,637 discloses polybutadiene compositions suitable for molded golf ball core construction. The compositions contain specific naturally occurring, essentially non-reinforcing, crystalline (preferably microcrystalline) silica particles, which have the effect of increasing the resilience and/or hardness of the resulting molded cores.
U.S. Pat. No. 5,886,074 discloses rubber compositions containing o-salicylsalicyclic acid of the formula:

U.S. Pat. No. 6,005,027 discloses blends of silanes and specific silicas, characterized in that when the blend is compounded into a rubber formulation even after four months of storage following formation of the blend, an unexpectedly high proportion of the silane present is available as a coupling and reinforcing agent for the rubber formulation. Over 80% and even over 90% of the silica can be extracted from the blend even four months after formation of the blend, and rubber formulation properties indicate high availability of the silane for coupling and reinforcement.
U.S. Pat. No. 6,028,143 discloses a rubber composition in which the coexistence of low heat build-up, heat resistance and high hardness can be achieved without damaging failure characteristics. A rubber composition comprising 100 parts by weight of matrix rubber and 2-75 parts by weight of a polyethylene composition which contains 0-80% by weight of polyethylene and 20% by weight or more of a composite comprising a polyethylene component and a rubber component previously bonded via a coupling agent to the polyethylene component, said rubber component is crosslinked with the matrix rubber. At least at one of the kneading stages before the final stage, the compound is kneaded so that the maximum temperature of the compound of the kneading of that stage is higher than the melting point of the polyethylene component mixed, preferably, by 10° C. or more.
U.S. Pat. No. 6,037,418 discloses a resin-reinforced elastomer which comprises as principal components, a rubber and a polyolefin having an average particle diameter of at most 1 micrometer, and in which the rubber and the polyolefin are linked together; a process for producing the resin-reinforced elastomer by melt kneading a polyolefin, a No.1 rubber and a binding agent to prepare a thermoplastic composition, adding a No.2 rubber to the resultant thermoplastic composition and melt kneading the mixture thus formed to disperse the polyolefin in the rubber components; and a pneumatic tire using the resin-reinforced elastomer in its tread. The above resin-reinforced elastomer is said to have a uniform modulus and a low density, to be minimized in directionality, and to have excellent tensile strength, fatigue resistance and abrasion resistance.
Australian Patent AU-A-10082/97 discloses the use in rubber silanes of the structure represented by R1nX3-nSi-(Alk)m(Ar)p—S(C═O)—R; where R1 is phenyl or alkyl; X is halogen, alkoxy, cycloalkoxy, acyloxy, or OH; Alk is alkyl; Ar is aryl; R is alkyl, alkenyl or aryl; n is 0 to 2; and m and p are each 0 or 1, but not both zero.
Japanese Patent Provisional Publication No. 186606/1995 discloses a pneumatic tire that comprises a rubber composition containing a thermoplastic resin in its tread rubber and that is improved in rolling resistance, while maintaining favorable traction characteristics.
Japanese Patent Publication No. 10204221 discloses studless tires comprising treads prepared by mixing hard rubber showing hardness (HD) at 0° of ≧70 with ≧40 parts of inorganic fillers per 100 parts of hard rubber, partly vulcanizing the compositions, particularizing the compositions to form particles (A), kneading matrix rubber having a glass transition temperature (Tg) of −40° C. with 5-50 parts of A particles per 100 parts of matrix rubber, and extruding the compositions to form treads containing A particles. A hard rubber composition with HD 70 and 50 phr ceramic particles with diameters of 150 (micrometers) were mixed, vulcanized, and particularized to form particles. A 70:30 blend of matrix rubber with a Tg of −40° C. and the particles were kneaded to give a tire tread showing skid resistance index (control tire 100) of 112 and abrasion resistance index of 99 on forming a tire comprising the tread.
Japanese Patent JP 63270751 A2 discloses the use of compounds represented by the general formula CH2═C(CH3)C(═O)S(CH2)1-6Si(OCH3)3 in tire tread compositions.
Korean Patent Publication No. 2000031877 discloses a rubber composition for tire treads said to have improved wear, driving, braking, and handling performance comprising 50 wt % styrene butadiene rubber (SBR) containing 23.5% styrene, 20-30 wt % SBR containing 40-50% styrene, 70-80 wt % carbon black, and 3-5 wt % toluol rosin, which is said to improve composition hardness, and additives. The synthetic rubber used comprises 59 wt % SBR with 23.5% styrene, 20-30 wt % styrene with 40-50 wt % styrene and 20-30 wt % BR, wherein the SBR content increases driving/braking performance, but decreases wear performance.
EP 894819 discloses a method for decreasing dynamic modulus without decreasing hardness in silica tread compounds in tires. The tread composition comprises an elastomer including a performance-enhancing package comprising silica, carbon black, and a silica replacement. The replacement replaces up to 40 percent by weight of the silica and yet maintains one or more selected performance properties as if the performance enhancement package were pure carbon black and silica. The replacement is kaolin clay present in conjunction with a silane coupling agent.
EP 964028 discloses compositions of triazoles, such as benzotriazole or tolyltriazole, in silica reinforced rubber compositions for tire components and tires made of such compounds. It is said that this results in high cure rate, cure efficiency, hardness, static and dynamic moduli without deleteriously affecting hysteresis. Methods for optimizing silica reinforced tire properties utilizing triazoles, brass powder and carbon black are also provided.
EP 1172406 discloses a rubber composition for a tire tread that comprises (a) a diene rubber, (b) glass fibers, (c) a reinforcing agent, and 1 to 15 parts by weight of (d-1) inorganic powders having a mohs hardness of less than 6.5 and an average particle size of less than 25 micrometers and/or (d-2) silicone rubber powders based on 100 parts by weight of the diene rubber.
WO99/22951 discloses a pneumatic tire that has belts, gum strips and an overlay that contains 2 to 15 phr thermoplastic reinforcing polymer as an addition to the compound, or as a replacement for 5 to 25 phr carbon black. The tire is said to have good high speed properties.